Currently, the best commercially available full contour (monolithic) zirconia dental ceramic materials are aesthetically inferior to lithium disilicate or leucite-based glass ceramic materials like IPS e.max or IPS Empress due to lower translucency and lack of opalescence. Better light transmittance and opalescence are required to better mimic natural dentition. Human enamel has varying “anisotropic” translucency which introduces many optical effects that are difficult to replicate with ceramic material. Opalescence is one optical characteristic of natural enamel that can create a highly complex visual display. To date, only glass ceramic materials come close to duplicating such optical complexity of natural dentition including opalescence. At the same time glass-ceramic materials are not as strong as zirconia materials hence limiting their clinical use to single- and multi-unit restorations and cases without bruxism.
U.S. Pat. No. 8,309,015, which is hereby incorporated by reference in its entirety, is directed to a method of processing tetragonal nanozirconia with grain sizes under 100 nm. The sintered body is claimed to only contain pores smaller than about 25 nm. The method is lacking bulk shape consolidation technology and does not address, mention or discuss opalescence. Rather, the requirements set forth in the patent and claims include the diameter of any pores which are present in the translucent zirconia sintered body to be not more than about 25 nm, which as believed, would preclude this material from being in the desired opalescent range as taught in the present invention and also is unrealistic for any practical bulk shape consolidation technology yielding dental articles via pressureless sintering.
U.S. Pat. No. 8,598,058, which is hereby incorporated by reference in its entirety, is directed to a method of processing nanozirconia articles with grain sizes under 200 nm and pore size under 50 nm comprising from about 0.5% to about 5.0% lanthanum oxide claimed to be essential to achieve the claimed properties. Again this patent does not address, mention or discuss opalescence despite showing sintered bodies illuminated with incident light whereby opalescence would be obvious if present.
U.S. Pat. Nos. 7,655,586 and 7,806,694, both hereby incorporated by reference in their entirety, are directed to a dental article and fabrication methods comprising: a single component yttria-stabilized tetragonal zirconia ceramic material having grains of average grain size exceeding 100 nanometers and not exceeding about 400 nanometers, wherein the ceramic material is fabricated of particulate material consisting essentially of ceramic crystallites with an average size of less than about 20 nm; wherein the particulate material is sintered without application of external pressure at a temperature less than about 1300° C. to a full density wherein the final pore size does not exceed the size of the ceramic crystallite size; and wherein the ceramic material exhibits at least 30% relative transmission of visible light when measured through a thickness of about 0.3 to about 0.5 mm. Again the requirements set forth in the patents and claims limit the diameter of pores and achievable grain size distributions which are present in the translucent zirconia sintered body, which as believed would preclude this material from being opalescent.
The following patents and published applications, directed to zirconia ceramics or processing methods, are hereby incorporated by reference in their entirety: U.S. Pat. Nos. 6,787,080, 7,655,586, 7,806,694, 7,833,621, 7,674,523, 7,429,422, 7,241,437, 6,376,590, 6,869,501, 8,298,329, 7,989,504, 8,425,809, 8,216,439, 8,309,015, 7,538,055, 4,758,541, US20110027742, US20120058883, US20100003630, US20090274993, US20090294357, US20090115084, US20110230340, US20090004098, US20100075170, US20040222098, and US20130313738. Among them U.S. Pat. No. 8,298,329 and US20130313738 describe translucent nano-crystalline dental ceramics and a process of fabrication of the same by slip-casting or powder compaction.
The following publications are directed to processing and properties of zirconia or transparent alumina ceramics.    Adam, J., et al. “Milling of Zirconia Nanoparticles in a Stirred Media Mill”, J. Am. Ceram. Soc., 91 [9] 2836-2843 (2008)    Alaniz, J. E., et al. “Optical Properties of Transparent Nanocrystalline Yttria Stabilized Zirconia”, Opt. Mater., 32, 62-68 (2009)    Anselmi-Tamburini, etc al. “Transparent Nanometric Cubic and Tetragonal Zirconia Obtained by High-Pressure Pulsed Electric Current Sintering”, Adv. Funct. Mater. 17, 3267-3273 (2007)    Apetz, R., et al. “Transparent Alumina: A Light Scattering Model”, J. Am. Ceram. Soc., 86 [3], 480-486 (2003)    Binner, J., et al. “Processing of Bulk Nanostructured Ceramics”, J. Eur. Ceram. Soc. 28, 1329-1339 (2008)    Binner, J. et al. “Compositional Effects in Nanostructured Yttria Partially Stabilized Zirconia” Int. J. Appl. Ceram. Tec., 8, 766-782 (2011)    Casolco, S. R. et al. “Transparent/translucent polycrystalline nanostructured yttria stabilized zirconia with varying colors” Scripta Mater. 58 [6], 516-519 (2007)    Garcia, et al. “Structural, Electronic, and Optical Properties of ZrO2 from Ab Initio Calculations”, J. Appl. Phys., 100 [1], 104103 (2006)    Klimke, et al. “Transparent Tetragonal Yttria-Stabilized Zirconia Ceramics” J. Am. Ceram. Soc., 94 [6] 1850-1858 (2011)    Knapp, K. “Understanding Zirconia Crown Esthetics and Optical Properties”, Inclusive Magazine, (2011)    Rignanese, et al, “First-principles Study of the Dynamical and Dielectric Properties of Tetragonal Zirconia” Phys. Rev. B, 64 [13], 134301 (2001)    Srdic, V. V., et al. “Sintering Behavior of Nanocrystalline Zirconia Prepared by Chemical Vapor Synthesis” J. Am. Ceram. Soc. 83 [4], 729-736 (2000)    Srdic, V. V., et al. “Sintering Behavior of Nanocrystalline Zirconia Doped with Alumina Prepared by Chemical Vapor Synthesis” J. Am. Ceram. Soc. 83 [8], 1853-1860 (2000)    Trunec, et al. “Compaction and Presureless Sintering of Zirconia Nanoparticles” J. Am. Ceram. Soc. 90 [9] 2735-2740 (2007)    Vladimir V. Srdic′, Markus Winterer, and Horst Hahn. “Sintering Behavior of Nanocrystalline Zirconia Prepared by Chemical Vapor Synthesis”. J. Am. Ceram. Soc., 83 [4] 729-36 (2000)
Most or all of the above-listed patents and publications describe a variety of properties of tetragonal nanozirconia materials and processing methods thereof. All of these sources appear to describe sintering with application of external pressure such as HIP or SPS.
Light transmission at about 550-560 nm is commonly accepted to compare light transmittance of dental materials, especially dental zirconia materials, which is related to the color resolution/sensitivity of photopic vision of human eyes. In humans, photopic vision allows color perception, mediated by cone cells in the retina. The human eye uses three types of cones to sense light in three bands of color. The biological pigments of the cones have maximum absorption values at wavelengths of about 420 nm (bluish-violet), 534 nm (Bluish-Green), and 564 nm (Yellowish-Green). Their sensitivity ranges overlap to provide vision throughout the visible spectrum from about 400 nm to about 700 nm. Colors are perceived when the cones are stimulated, and the color perceived depends on how much each type of cone is stimulated. The eye is most sensitive to green light (555 nm) because green stimulates two of the three kinds of cones almost equally; hence light transmission at 560 nm is used as a basis for characterization of highly translucent zirconia materials of the present invention.
Opalescence is one of the important optical characteristics of natural dentition that is critical to replicate in aesthetic dental restorative material in order to fabricate life-like dental restorations. This esthetic requirement is often referred to as the “vitality of a restoration”. It is a well-known optical effect resulting in a bluish appearance in reflected color and an orange/brown appearance in transmitted color. The opalescent property is generally associated with scattering of the shorter wavelengths of the visible spectrum, by inclusions of the second phase(s) having a different refractive index from the matrix phase. In human teeth, opalescence of natural enamel is related to light scattering and dispersion produced by complex spatial organization of enamel's elemental constituents—hydroxyapatite nanocrystals. Hydroxyapatite crystallites forming human enamel are arranged in bundles or sheets forming rods (bundles) and interrods (sheets), which are organized in a honeycomb-like structure. The average crystal size is 160 nm long and 20-40 nm wide. As light travels through the enamel, the rods scatter and transmit the shorter wavelength light, rendering the enamel opalescent.
The degree of opalescence can be quantified by a colorimetric spectrophotometry measurement with a CIE standard. For example, Lee et al. (see references below) use “Opalescence Parameter” (OP) as a measure of opalescence. Kobashigawa et. al. (U.S. Pat. No. 6,232,367) use the same basic formula, but termed it “Chromaticity Difference”. The opalescence parameter (OP or “Chromaticity Difference”) is calculated according to the following formula:OP=[(CIEaT*CIEaR*)2+(CIEbT*CIEbR*)2]1/2,wherein (CIEaT*−CIEaR*) is the difference between transmission and reflectance modes in red-green coordinate a*; (CIEbT*−CIEbR*) is the difference between transmission and reflectance modes in yellow-blue color coordinate b*. Using this formula, OP of the commercially available current state of the art “translucent” zirconia is calculated to be in the range from about 5 to about 7. These commercial materials are clearly not opalescent. According to literature data, it is believed that materials with low OP values are not opalescent. The measured OP range for clearly opalescent human enamel was 19.8-27.6. According to Kobashigawa, for matching the vitality of natural teeth, the OP value should be at least 9, and preferably higher, so that the opalescence effect is clearly observed. On the other hand it is not useful to match high OP values of human enamel “just by numbers” since the restoration will not blend well with the adjacent teeth in the patient's mouth.
The following publications are directed to mechanisms of opalescence in natural or synthetic materials.    Cho, M.-S. et al. “Opalescence of all-ceramic core and veneer materials”, Dental Materials, 25, 695-702, (2009)    Egen, M. et al. “Artificial Opals as Effect Pigments in Clear-Coatings”, Macromol. Mater. Eng. 289, 158-163, (2004)    Lee, Y.-K., et al. “Measurement of Opalescence of Resin Composites”, Dental Materials 21, 1068-1074, (2005)    Lee, Y.-K., et al. “Changes in Opalescence and Fluorescence Properties of Resin Composites after Accelerated Aging”, Dental Materials 22, 653-660, (2006)    Lee, Y.-K., “Influence of Scattering/Absorption Characteristics on the Color of Resin Composites” Dental Materials 23, 124-131, (2007)    Lee, Y.-K., “Measurement of Opalescence of Tooth Enamel”, Journal of Dentistry 35, 690-694, (2007)    Kobashigawa, A. I. et al., “Opalescent Fillers for Dental Restorative Composites”, U.S. Pat. No. 6,232,367 B1, (2001)    Peelen. J. G. J. et al. “Light Scattering by Pores in Polycrystalline Materials: Transmission Properties of Alumina”, Journal of Applied Physics, 45, 216-220, (1974)    Primus, C. M., et al. “Opalescence of Dental Porcelain Enamels” Quintessence International, 33, 439-449, (2002)    Yu, B., et al. “Difference in Opalescence of Restorative Materials by the Illuminant”, Dental Materials 25, 1014-1021, (2009)    White et al., Biological Organization of Hydroxyapatite Crystallites into a Fibrous Continuum Toughens and Controls Anisotropy in Human Enamel, J Dent Res 80(1): 321-326, (2001).
It would be extremely beneficial to have high translucency of glass ceramics combined with high strength of tetragonal zirconia and opalescence mimicking natural dentition in the same dental restorative material sinterable below 1200° C., which can be processed into a full contour zirconia restoration using conventional techniques and equipment such as dental CAD/CAM systems, dental pressing furnaces and dental furnaces. Other techniques and equipment successfully used in other areas of technology for mass production of near-net shaped parts and components can be also used.